1. Field of the Invention
The invention relates to wireless location systems, and more particularly, to enhanced precision in location estimation of a mobile station under different environments.
2. Description of the Related Art
Mobile location estimation is of considerable interest in wireless communications. A mobile station (MS) may locate itself by communicating with a plurality of geometrically distributed base stations (BS).
FIG. 1 shows a Time-of-Arrival (TOA) based location estimation with three base stations. For a lth BS, TOA tl is estimated as:
                                          t            l                    =                                                    r                l                            c                        =                                                                                ζ                    l                                    c                                +                                                      n                    l                                    ⁢                                                                          ⁢                  l                                            =              1                                      ,        2        ,        …        ⁢                                  ,        N                            (        1        )            
Where c is the speed of light, rl represents the measured relative distance between the mobile station (MS) and lth BS, composed of actual distance ζl and TOA measurement noise nl. The actual distance ζl can be obtained according to the formula:ζl=√{square root over ((x−xl)2+(y−yl)2)}{square root over ((x−xl)2+(y−yl)2)}  (2)
Where the coordinates (x, y) represents the MS's location to be determined, and (xl, yl) is the location of lth BS.
In FIG. 1, with the measured distances rl used as radiuses, three circles BS1, BS2 and BS3 are correspondingly formed for each lth BS. Ideally, the measured distance rl meets the actual distance ζl, thus three circles intersect at the single point (x,y) where the MS is located. Due to None-Line of Sight (NLOS) measurement errors, however, the measured distance rl is always larger than the actual distance ζl, and a rough, or confined region, defined by cross points A, B and C is respectively formed instead, thus, the MS is theoretically situated somewhere in the defined region. In X. Wang, Z Wang and B. O'Dea “A TOA-based location algorithm reducing the errors due to non-line-of-sight (NLOS) propagation” Published in IEEE Trans., vol. 52, January 2003, a two step least square (LS) algorithm is utilized to converge the estimated MS location (xe, ye) to the actual MS location (x,y). For gentle NLOS environments, the two-step LS algorithm is efficient and highly accurate. When the confined region ABC grows as the NLOS error increases, the accuracy of the two-step LS algorithm may significantly decrease, particularly for an MS located at the boundaries of arcs AB, BC and CA. Thus, an improved algorithm is desirable.